Add exception handler overloads to ObserveDuration

[phnx47]

Close: #183

Add onObserveException overloads to ObserveDuration and ObserveDurationAsync, allowing exceptions from Observe to be handled separately from exceptions thrown by the measured code.

• New overloads accept Action<Exception> onObserveException which is called only when Observe throws
• Exceptions from the user-provided method always propagate normally
• Existing overloads unchanged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (9f9a6da) to head (cc4dd2d).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
+ Coverage   88.83%   90.33%   +1.50%     
==========================================
  Files          48       48              
  Lines        1738     1770      +32     
  Branches      233      234       +1     
==========================================
+ Hits         1544     1599      +55     
+ Misses        141      119      -22     
+ Partials       53       52       -1     

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[Copilot]

Pull request overview

This PR extends IValueObserver duration-measurement extension methods to optionally route exceptions thrown by Observe(...) to a caller-provided exceptionHandler, while allowing exceptions from the measured user code to propagate normally.

Changes:

• Added exceptionHandler overloads for ObserveDuration / ObserveDurationAsync (sync + generic, async + generic).
• Introduced a shared helper to perform Observe(...) with optional exception handling.
• Added a new test suite covering success paths, method-exception paths, and observe-exception handling behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Prometheus.Client.Abstractions/ValueObserverExtensions.cs	Adds new overloads and a helper to handle exceptions thrown by Observe(...) separately from the measured method.
tests/Prometheus.Client.Tests/ValueObserverExtensionsTests.cs	Adds comprehensive tests for the new overloads and preserves existing behavior expectations.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

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